1. Field of the Invention
This invention relates generally to centrifugal separators for separating mixtures of liquid and solid particles, and more particularly concerns a continuous centrifugal separating apparatus for recovery of components of a slurry containing finely divided, suspended particles.
2. Description of Related Art
Semiconductor components are commonly manufactured by layering electrically conductive and dielectric materials to achieve appropriate electrical characteristics for fabrication of multiple electrical components such as resistors, capacitors and transistors. Many of these discrete devices are incorporated into integrated circuits for use in creating microprocessors, memory chips, logic circuits, and the like. Many integrated circuits can be produced on semiconductor wafers by layering of dielectric and electrically conductive materials to create multiple semiconductor devices in a relatively small area.
The density of electrical components and wiring on such semiconductor devices have continually increased as trace line widths on such semiconductor devices have narrowed. At one time, for example, trace line widths on such devices typically ranged from 1 .mu.m to 4 .mu.m. However, in recent years, the industry has made significant advances in reducing trace line widths used in creation of integrated circuits to less than 1 .mu.m. Currently, trace line widths of 0.5 to 0.35 .mu.m are common, and research is being done to achieve trace line widths of from 0.25 .mu.m to 0.18 .mu.m. In addition, the demand for increased memory and computing power has driven limits on the number of semiconductor devices per integrated circuit ever higher, resulting in an increase in the number of layers applied to semiconductor wafers, while the typical size of the integrated circuits continues to decrease. The combination of narrower trace line widths, increased layers of materials and higher densities of semiconductor devices per integrated circuit has made such devices increasingly susceptible to failure due to inconsistencies on semiconductor wafer surfaces, and it has become increasingly important that such semiconductor wafers have surfaces and dielectric layers that are uniformly smooth.
Methods for chemical and mechanical planarization (CMP) have been developed to polish the surface of semiconductor wafers, and typically involve rotating the wafer on a polishing pad, applying pressure through a rotating chuck, and supplying an aqueous chemical slurry containing an abrasive polishing agent to the polishing pad for both surfactant and abrasive action. The chemical slurry can additionally contain chemicals that etch various surfaces of the wafer during processing. Abrasive agents that can be used in the chemical mechanical slurry include particles of fumed silica, cesium and alumina. The chemical mechanical slurry can also include stabilizer or oxidizer agents. Fumed silica is typically mixed with a stabilizer such as potassium hydroxide or ammonium hydroxide, and is commonly used to polish dielectric or oxide layers on the semiconductor wafer. Cesium and alumina are commonly mixed with an oxidizer agent such as ferric nitrate or hydrogen peroxide, and are typically used to polish metal layers, such as tungsten, copper and aluminum, for example.
The slurry and material removed from the various layers of the semiconductor wafer form a waste stream that is commonly disposed of as industrial waste. The abrasive components constitute approximately 8% to 15% of this waste stream, with the remainder constituting other chemical agents such as stabilizer or oxidizer agents, and water. The waste stream is typically diluted with rinse water to yield a final solids concentration of approximately 1% to 1.5%in the waste stream. These solids are finely divided and composed of a variety of diameters and densities, ranging all the way down to the sub micron range. Particularly in light of recent environmental concerns, it would be desirable to provide a process and apparatus to remove abrasive components from the waste stream for reuse in the chemical mechanical slurry or for other purposes. It would also be desirable to treat and reclaim the waste stream supernatant liquid to permit reuse of the supernatant liquid from the chemical mechanical planarization process.
In order to accomplish these goals, the inventors have developed a process and apparatus to effectively recover the CMP waste components and liquids for reuse. The process and apparatus for performing the process is described in a co-pending application in corporated by reference herein, Ser. No. 08/870,082 filed Jun. 5, 1997, entitled "Method And Apparatus For Recovery Of Water And Slurry Abrasives Used For Chemical And Mechanical Planarization." The present application is for a component of that system, a continuous centrifugal separator. Such a continuous centrifugal separator has important benefits to the system, which does continuous, rather than batch, processing.
Centrifuges are commonly used for separating heterogeneous liquid-liquid or liquid-solid mixtures of different specific gravities and which are not soluble in one another. The physical principle on which the functioning of a centrifuge is based can be explained by first considering what happens when grains of sand suspended in water settle to the bottom. When sand and water in a jar is stirred up, the force of gravity will attract the grains of sand more strongly than the water molecules. The sand grains are therefore rapidly pulled down to the bottom of the jar, in a process described as sedimentation. After a time, the system comes to rest, and two layers will have formed in the jar, with a layer of water over a layer of sand on the bottom of the jar. A similar separation can be performed in a centrifuge, subjecting materials to be separated to a greater centrifugal force instead of gravitational force, allowing separation of materials with different densities in a shorter period of time. The magnitude of the centrifugal force can be increased as desired, and is dependent on the speed of rotation of the centrifuge and the geometry of the fluid passages.
Batch centrifugal separation can be used to separate solids particles ranging in size from the sub-micron level to very large particle sizes. The efficacy of batch centrifugal separators is based on the speed of rotation, the angle of the separation cylinder, and the time that the sample is subjected to high centrifugal forces. Batch centrifugal separators generally can be quite efficient in separating solids from liquids. Continuous centrifugal separators are typically used for separating larger quantities of mixtures; however, continuous centrifugal separators are typically designed to handle larger particle sizes in the range of 5 .mu.m and greater, although some have accomplished continuous separation of particles in the 1 .mu.m to 2 .mu.m range. Conventional continuous centrifugal separators are generally inefficient at separating particles in the sub micron range of 100 nm.
The need exists for a continuous centrifugal separator which can be used in combination with a process relying on electrophoresis and agglomeration techniques to progressively remove particulates from a CMP process. Such a centrifugal separator must be robust, reliable and capable of reclaiming sub micron particles from the liquid base of the incoming waste materials. The present invention satisfies those needs.